Engine valves

ABSTRACT

An engine valve according to the present teaching has a hot-forged head portion having an oxidized surface layer at a radially outer peripheral surface. The oxidized surface layer is formed during hot-forging of the head portion.

This application claims priority to Japanese patent application serial number 2009-292502, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present teaching relates to engine valves made of heat-resistant alloy and having a valve head portion that is hot-forged in continuity with one end of a shaft portion.

2. Description of the Related Art

A process of manufacturing this kind of poppet-type engine valve is hereinafter described. As shown in FIG. 3 (a), one end of a bar member is cold-forged to form a valve intermediate body 106 having a shaft portion 102 and a head portion primary body 105 having a block-like shape and formed in series at one end of the shaft portion 102. The valve intermediate body 106 is then inserted into a casting mold 110 and heat-forged in a manner that a punch 111 presses against the head portion primary body 105. In this way, the head portion primary body 105 is cast into a head portion 103 that fits to the shape of the casting mold 110. Conventionally, in order to ensure that the head original body 105 becomes a net shape fitting accurately to the shape of the mold 110, the forging process is made in such a manner that a burr 107 is formed around the outer peripheral edge of the head portion 103. More specifically, the volume of the head portion primary body 105 is determined to be slightly larger than the volume defined by a head portion mold surface 110 a of the casting mold 110 and a bottom surface of the punch 111. Therefore, the burr 107 is inevitably formed at the outer peripheral edge of the head portion 103 after the hot-forging process. The burr 107 is then removed by a machining operation such as a grinding or cutting operation. Examples of engine valves manufactured in this way are disclosed in Japanese Laid-Open Patent Publication Nos. 3-86457 and 4-8402.

When the heat resistant alloy is hot-forged, the base material is oxidized to form an oxidized layer on a surface layer portion of the valve intermediate body. The oxidized layer has a favorable heat insulation property. Therefore, in the case of an engine valve that is exposed to a high temperature environment and receives a high thermal load, it is preferable that the oxidized layer is left without being removed. Due to the heat insulation property of the oxidized layer formed in the surface layer portion, the amount of heat transmitted into the engine valve is reduced and increase of temperature of the engine valve is suppressed. Especially, the heat insulation effect in the head portion is significant because the head portion is easily heated to have a high temperature as it is directly opposed to a combustion chamber of the engine.

However, in the case of Japanese Laid-Open Patent Publication Nos. 3-86457 and 4-8402, the burr produced around the outer peripheral edge of the head portion is removed by a machining operation. Therefore, even if an oxidized layer 101 b is formed at the surface layer of the base material 101 a by the heat-forging process, it may be removed at an outer peripheral surface 103 d of the head portion 103 by the machining operation as shown in FIG. 4. Under the circumstance where the oxidized layer 101 b is removed, the outer peripheral surface 103 d of the head portion 103 is easily heated. Accordingly, the possibility of occurrence of “pre-ignition phenomenon” may be increased. Here, the term “pre-ignition phenomenon” is used to mean a phenomenon in which timing of combustion is advanced prior to controlled ignition by a sparking plug because air-fuel mixture in the combustion chamber may be combusted by the highly heated engine valve.

In order to avoid the pre-ignition, an oxidized layer may be formed again on the outer periphery surface (machined surface) after the burr has been removed from the head portion of the engine valve as disclosed in Japanese Laid-Open Patent Publication Nos. 3-86457 and 4-8402. However, the re-forming process is troublesome and leads to increase of the cost.

Therefore, there is a need in the art for an engine valve that is difficult to be heated due to a high heat insulation property of an oxidized layer formed on an outer peripheral surface of a head portion.

SUMMARY OF INVENTION

An engine valve according to the present teaching has a hot-forged head portion having an oxidized surface layer at a radially outer peripheral surface. The oxidized surface layer is formed during hot-forging of the head portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an engine valve according to an example;

FIGS. 2( a) and 2(b) are views showing manufacturing steps of the engine valve;

FIGS. 3( a) and 3(b) are views showing manufacturing steps of a known engine valve; and

FIG. 4 is a sectional view of the known engine valve.

DETAILED DESCRIPTION OF THE INVENTION

Each of the additional features and teachings disclosed above and below may be utilized separately or in conjunction with other features and teachings to provide improved engine valves. Representative examples of the present invention, which examples utilize many of these additional features and teachings both separately and in conjunction with one another, will now be described in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Moreover, various features of the representative examples and the dependent claims may be combined in ways that are not specifically enumerated in order to provide additional useful embodiments of the present teachings.

In one example, an engine valve includes a shaft portion and a head portion each made of heat resistant alloy. The head portion is disposed at one end of the shaft portion in continuity therewith and hot-forged without forming any burr at a radially outer peripheral surface (marginal surface) of the head portion. The outer peripheral surface of the head portion is not machined so that the outer peripheral surface of the head portion has an oxidized surface layer formed by oxidization of the heat-resistant alloy. The valve face of the head portion contacting the valve seat may be ground for removing the oxidized surface layer if necessary or desired. The radially outer peripheral surface of the head portion is a surface extending between the flat head surface for facing to) a combustion chamber and the valve face for contacting the valve seat.

If no burr is formed when the head portion is forged, the machining operation such as a cutting or grinding operation to remove the burr is not required. Therefore, the radially outer peripheral surface of the head portion is remained as it is forged. More specifically, the oxidized layer formed by oxidization of the surface layer of the base material during the hot-forging process is not necessary to be removed by the machining operation. In this way, the oxidized layer may be maintained on the outer periphery surface of the head portion. Because the oxidized layer having high heat insulation property is remained also at the outer peripheral surface of the head portion, increase of temperature of the head portion, which is easily heated to a high temperature, can be prevented. Accordingly, it is possible to lower the temperature of the engine valve (especially the head portion) during operation of the engine compared to the known techniques as disclosed in Japanese Laid-Open Patent Publication Nos. 3-86457 and 4-8402. In this way, a critical region for occurrence of pre-ignition is improved. If the critical region for occurrence of pre-ignition is improved, combustion of the fuel mixture within the engine is stabilized. Accordingly, an engine output and fuel consumption can also be improved by increasing the compression ratio of the fuel mixture. Further, because the oxidized layer formed during the hot-forging process is simply maintained, it is not necessary to form the oxidized layer again after the machining operation or it is not necessary to form a heat insulation layer separately. Accordingly, the engine valve can be manufactured efficiently and at a lower cost.

Preferably, the outer peripheral surface of the head portion has a curved configuration. Because a high heat insulation property is ensured, it may be possible that the outer peripheral surface is pointed. However, if the surface is curved, occurrence of the pre-ignition of the air-fuel mixture may be more effectively prevented compared to the pointed surface As a heat-resistant alloy, a heat-resistant steel may be used.

An example of an engine valve of the present teaching is explained below with reference to the drawings.

As shown in FIG. 1, a poppet-type engine valve 1 has a bar-shaped shaft portion 2 and a head portion 3 having a configuration enlarged in a direction away from the shaft portion 2. The head portion 3 is formed in continuity with on one end of the shaft portion 2. The engine valve 1 may be used for introducing air into a combustion chamber of an internal combustion engine (hereinafter simply called “engine”) or for discharging exhaust gas from the combustion chamber. Thus, the engine valve 1 may be used as an intake valve for introducing air-fuel mixture into the combustion chamber or as an exhaust valve for discharging exhaust gas produced after combustion of the mixture in the combustion chamber. Although not shown in the drawings, when the engine valve 1 is mounted on the engine, the other end of the shaft portion 2 may be pressed by a cam, a pivot, and a locker arm, etc., so that the engine valve 1 moves away from a valve seat to open an intake port or an exhaust port of the combustion chamber.

The engine valve 1 is made of heat-resistant alloy. For example, heat-resistant alloy may be heat-resistant steel, such as SUH3, SUH11 or SUH35 (in JIS classification). Alternatively, titanium alloy may be used as the heat-resistant alloy. The head portion 3 includes a flat head surface 3 a, a valve face 3 b and a neck portion 3 c. The flat head surface 3 a may face to the combustion chamber. The valve face 3 b can contact the valve seat in order to close the intake port or the exhaust port of the combustion chamber. The neck portion 3 c is formed in series from the face surface 3 b to a shaft portion 2. The diameter of the neck portion 3 c is gradually reduced towards the shaft portion 2. The head surface 3 a and the face surface 3 b are connected to each other via a radially outer peripheral surface 3 d of the head portion 3. The outer peripheral surface 3 d of the head portion 3 is configured as a curved surface as it is not machined. The engine valve 1 has an oxidized surface layer 1 b formed throughout on its outer surface. The oxidized surface layer 1 b is formed due to oxidization of a base material 1 a of the engine valve 1. In particular, the outer peripheral surface 3 d of the head portion 3 is covered with the oxidized layer 1 b.

A manufacturing process of the engine valve 1 will be explained below with reference to FIGS. 2( a) and 2(b). First, a rod made of the heat-resistant alloy is cold-forged or hot-forged, for example, by drawing the rod with a four-side high-speed forging apparatus or is upset-forged by an electric upset-forging apparatus (i.e. an electric upsetter), so that a valve intermediate body 6 (intermediate product) is formed as shown in FIG. 2( a). As shown in FIG. 2( a), the valve intermediate body 6 includes a massive head portion primary body 5 formed in continuity with one end of the shaft portion 2. The valve intermediate body 6 is inserted into a forging die 10 as shown in FIG. 2( a). The volume of the head portion primary body 5 is set to be slightly smaller than a volume of a head portion to be forged (hereinafter called “head portion volume”), which is defined between a mold surface 10 a of the forging die 10 and a bottom surface of a punch 11 when the punch 11 is positioned at its lowermost position. Then, the head portion primary body 5 is hot-forged by being punched several times from above by the punch 11 that reciprocates between its uppermost position and lowermost position by a predetermined stroke. In this way, the head portion primary body 5 is plastically deformed. Accordingly, the head portion primary body 5 is forged into the head portion 3 having a configuration corresponding to the shape of the head portion volume of the forging die 10. Because the volume of the head portion primary body 5 is slightly smaller than the head portion volume defined by the forging die 10, no burr is formed at the radially outer peripheral surface 3 d of the head portion 3, and therefore, the radially outer peripheral surface 3 d maintains a curved configuration given as a result of plastic deformation. When the punch 11 is at the lowermost position, the punch 11 does not contact the forging die 10 and thus, there is a slight clearance between the punch 11 and the forging die 10.

The hot-forging process of the head portion primary body 5 may be made at a temperature of 800° C. or more. The heat may only be applied to the head portion primary body 5 to be plastically deformed. Due to the hot-forging of the head portion primary body 5, the oxidized layer 1 b is formed on the surface of the engine valve 1 as a result of oxidization of the base material 1 a (e.g., heat resistant steel). In the present teaching, the burr is not formed at the radially outer peripheral surface 3 d of the head portion 3 after the hot-forging process. Accordingly, it is not necessary to remove the burr from the outer peripheral surface 3 d of the head portion 3 by a machining operation, such as grinding or cutting. In this way, the engine valve 1 having the oxidized layer 1 b formed by the hot-forging process also at the outer peripheral surface 3 d of the head portion 3 may be obtained. Because the oxidized layer 1 b having a high heat insulation property is formed at the outer peripheral surface 3 d of the head portion 3, any heat that may be applied to the outer peripheral surface 3 d of the head portion 3 during the operation of the engine can be effectively insulated. Therefore, increase in temperature of the head portion 3 is also well prevented compared to the known engine valve. Accordingly, it is possible to improve a critical region for occurrence of pre-ignition.

The valve face 3 b may be ground if necessary or desired. In addition to the oxidized layer 1 b, another kind of heat insulation layer (a heat insulation film) may be formed on the surface of the engine valve 1. A nitriding treatment such as a soft-nitriding treatment in a salt bath and a gas soft-nitriding treatment may also be applied to the surface of the engine valve 1. As a material of an additional heat insulation layer, ceramic series oxide, such as alumina, cordierite, zirconia, zircon, oxidized titanium and magnesia; ceramic series carbide, such as silicon carbide; and ceramic series nitride, such as silicon nitride may be used. In addition to these materials, ceramic series materials having high heat resistance and heat insulation property, such as aluminum silicate, oxidized chrome, WC/Co alloy, WC/Ni/W/Cr3C2 alloy, and Cr3C2/Ni—Cr alloy are also appropriate to be used for the heat insulation layer. The heat insulation layer could be used as a single layer or a plurality of heat insulation layers made of different materials maybe layered. The heat insulation layer may be formed by using a suitable technique, such as gas flaring, arc spraying, plasma spraying, blast spraying, sputtering and ion plating techniques. Furthermore, it may be desirable to anneal the heat insulation layer at a low temperature, for instance, approximately at 400° C. for removing residual stress generated during the forging process. 

1. An engine valve comprising: a shaft portion and a head portion each made of heat resistant alloy; wherein the head portion is disposed at one end of the shaft portion in continuity therewith and hot-forged without forming any burr at a radially outer peripheral surface of the head portion; wherein the radially outer peripheral surface of the head portion is not machined so that the radially outer peripheral surface of the head portion has an oxidized surface layer formed by oxidization of the heat-resistant alloy.
 2. The engine valve as in claim 1, wherein the radially outer peripheral surface of the head portion has a curved configuration.
 3. The engine valve as in claim 1, wherein the heat-resistant alloy is heat-resistant steel.
 4. The engine valve as in claim 1, wherein the heat-resistant alloy is titanium alloy.
 5. The engine valve as in claim 1, wherein a surface of the engine valve includes a flat head surface, a valve face formed in continuity with the head surface via the radially outer peripheral surface, and a neck surface having a diameter decreasing toward the shaft portion.
 6. A method of manufacturing an engine valve, comprising: preparing a rod made of heat resistant alloy; forming the rod into an intermediate product having a shaft portion and a head portion primary body; and hot-forging the head portion primary body into a head portion, so that an oxidized surface layer is formed on a surface of the head portion without producing any burr at a radially outer peripheral surface of the head portion.
 7. The method as in claim 6, wherein the step of forming the rod into the intermediate product comprises forging a portion of the rod into the head portion primary body. 